Electrical make and break contact



I Patented Nov. 21,1939

UNITED STATES ELECTRICAL MAKE AND BREAK CONTACT Franz,R. House] and Kenneth L. Emmett, In-- ,dianapolis, Ind., assignors to P. R. Mallory & (70., Inc., Indianapolis, Ind., a corporation of Delaware No Drawing. Application January 5, 1939,

Serial No. 249,426

4 Claims.

The present invention relates to electrical make and break contacts.

An object of the invention is to improve the characteristics of the electrical contact elements 5 containing silver.

Another object of the invention is to provide a silver base contact material which will combine a low contact resistance with a low material transfer.

A still further object is to provide a new contact, material which can be used under severe electrical loads without welding or sticking.

Other objects of the invention will be apparent from the following description, taken in con- 5 nection with the appended, claims.

The present invention comprises the combination of elements, methods of manufacture and.

. products thereof, brought out and exemplified in the disclosure hereinafter set forth, the scope of the invention being indicated in the appended claims.

While a preferred embodiment of the invention is described herein, it is contemplated that considerable variation may be made in the method of procedure ,and combination of elements,-

without departing from the spirit of the invention.

The present invention comprises an improvement in silver base electric contact elements. According to the invention, the contacts are improved by the addition of metallic thallium.

The contacts may be composed of silver and metallic thallium accordingly. The ingredients may be present in approximately the following' proportions:

Up to that percentage, silver and thallium will form homogeneous or quasi-homogeneous mixtures. In this respect, thallium, when alloyed with silver, behaves quite differently from lead, which is present in the form of free lead particles in the silver matrix. These lead particles .make it impossible to work either hot or cold any alloys containing appreciable percentages of lead. Silver-thallium alloys, with lower percentages of thallium, can be Worked satisfac torily into rod or wire, provided that the percentage of cold reductions is not excessive. It is likewise possible to improve the characteristics of silver alloys containing additional ingredients, by the addition of metallic thallium.

Thus, silver alloys containing beryllium, tin,

zinc, cadmium, magnesium, copper, gold, nickel, manganese, palladium group elements, platinum group elements, silicon, titanium, zirconium, indium, uranium, thorium, arsenic and antimony, can be improved by the addition of metallic thal- 5 lium in the substantial proportions indicated above.

The alloy of silver and thallium with or without other additives can be readily prepared simply by melting the ingredients together accord- 10 mg to well-known alloying procedures.

We have prepared contacts having various percentages of metallic thallium within the range specified. The compositions were tested on a resistive inductive circuit at 470 cycles per minute, and wherein the current flowing in the circuit was increased periodically to obtain the definite current values at which failure by sticking 'would occur.

The amount of material transfer from one contact to another was also deter- 20 mined. It was found that an alloy containing 3% metallic thallium would stand a critical current of approximately 11.5 amperes and would show a transfer of .008". By comparing this alloy with a silver base material containing 3% 25 lead, the following data were obtained for th silver-lead combination: I

Critical current 1 9 amps. Build-up .013" a The 3% silver-thallium alloy showed an elec trical conductivity of approximately 30% International Annealed Copper Standard and a specific gravity of 10.5 grams per c. c. The thermal conductivity of the material is similarly high and 35 therefore will prevent the Contact from overheating, oxidizing or welding. The fact that the alloy can be cold worked makes it possible to obtain a harder contact surface which will withstand mechanical wear better than a lead con- 40 taining silver alloy, which is generally being used in the as cast condition.

It has been found experimentally that the combinations of the present'invention maintain the low contact resistance, and at the same time 45 a desirable low factor of material transfer. Such transfer as does take place results in a deposit in the form of a dome as differentiated from the undesirable needle or peaked type. It should be further brought out that contacts manufactured 50 in accordance with the prior art, through their tendency to show exaggerated transfer, maintain an erratic resistance characteristic which on occasion, may be lower than the average value of compositions covered by the present inven- 55 tion. This periodic drop in resistance of contacts of prior art, is caused by the transfer periodically penetrating the oxide layer or' film, or by a definite increase in effective contact pressure, due to reduced contact area, brought about by the material transfer.

The application of contacts of the prior art to electrical devices necessitates in many instances,'

make contacts of the prior art generally applicable to sensitive relays or selector switches, in that it is impossible to develop sufilcient pressure without materially affecting the operation of the unit. The material covered by the present invention through its resistance to material transfer and'its low average contact resistance, is highly desirable for equipment of this type.

The alloys of the present invention also can I be prepared in the form of contact bimetals or bimetallic contacts in whichone side consists of base metal.

While the present invention as to its objects and advantages has been described herein as car .spirit and scope of the appended claims.

What is claimed is:

l. An-electrical contacting member, formed of an alloy composed predominantly of silver, to which has been added metallic thallium, in quantities ranging from .05 to 15%.

2. An electric contact member formed of. an alloy of .1 to 7% of metallic thallium, balance substantially all silver.

3. An electric contact formed of an alloy of about 3% thallium, balance silver. v

4. An electric contact member formed of a silver-base alloy containing from about .05% to 15% thallium and characterized by uniformity of contact resistance over long periods of operation and by low tendency to transfer material during contact operation and freedom from needle-like contact material build-up during operation.

FRANZ R. HENSEL. KENNETH L. EMMERT. 

